1. Field of the Invention
The present invention relates generally to the field cf muscle stimulation, and more particularly, relates to monitoring of cardiac assist systems which are powered by skeletal muscle.
2. Description of the Prior Art
Cardiac assist systems do not replace the human heart but merely supplement it. Many techniques have been proposed using a variety of mechanical power sources. Typically these require some form of percutaneous energy transfer, because of the difficulty in storing sufficient energy subcutaneously. Such systems are cumbersome and inconvenient for the patient and are prone to infection along the percutaneous energy transfer path.
A technique holding a great deal of promise is to power the cardiac assist system from a surgically modified skeletal muscle. The cardiac assist system is thus powered by normal biochemical processes. U.S. Pat. No. 4,813,952 issued to Khalafalla teaches a number of configurations of a skeletal muscle powered cardiac assist system. U.S. Pat. No. 4,411,268 issued to Cox teaches a technique for conditioning skeletal muscle to be used in such a cardiac assist system.
Whereas the feasibility of a skeletal muscle powered cardiac assist system has been established in the literature and the clinic, a practical system must address concerns regarding efficiency and safety of operation. Of specific concern is the tying of the rate of stimulation of the skeletal muscle directly to the heart rate. This seems appropriate in some instances, but care must be exercised because of the wide range of possible rates. For example, it may be quite inefficient to stimulate the skeletal muscle at the cardiac rate when the patient is at rest and requires only modest cardiac output. Similarly, it may be inefficient and even dangerous to stimulate skeletal muscle contraction at very high rates. The nature of the skeletal muscle stimulation may also be changed to improve efficiency over the range of available rates and cardiac demands.